Sex over Food the Clear Choice for Some Males

Researchers from the University of Rochester say they have shown that male brains—at least in nematodes—will suppress the ability to locate food in order to instead focus on finding a mate. Their study (“Sex, Age, and Hunger Regulate Behavioral Prioritization through Dynamic Modulation of Chemoreceptor Expression”), which appears in Current Biology, may point to how subtle changes in the brain’s circuitry dictate differences in behavior between males and females.

“While we know that human behavior is influenced by numerous factors, including cultural and social norms, these findings point to basic biological mechanisms that may not only help explain some differences in behavior between males and females, but why different sexes may be more susceptible to certain neurological disorders,” said Douglas Portman, Ph.D., an associate professor in the department of biomedical genetics and Center for Neural Development and Disease at the University of Rochester and lead author of the report.

C. elegans is particularly useful in the study of the nervous system and scientists understand in great detail the development, function, and multiple connections of its entire neural network. The current study focuses on the activity of a single pair of neurons found in C. elegans—called AWA—that control smell. Smell, along with taste and touch, are critical sensory factors that dictate how C. elegans understands and navigates its environment, including finding food, avoiding danger, and locating a mate.

There are two sexes of C. elegans, males and hermaphrodites. Though the hermaphrodites are able to self-fertilize, they are also mating partners for males, and are considered to be modified females. It has been previously observed that males and hermaphrodites act differently when exposed to food. If placed at a food source, the hermaphrodites tend to stay there. Males, however, will leave a food source and wander. Scientists believe they do this because they are in search of a mate.

The Rochester researchers discovered that the chemoreceptors of the AWA neurons were regulated by the sexual identity of these cells, which, in turn, controls the expression of a receptor called ODR-10. These receptors bind to a chemical scent that is given off by food and other substances.

In hermaphrodites, more of the ODR-10 receptors are produced, making the worms more sensitive, and thereby attracted, to the presence of food. In males, fewer of these receptors are active, essentially suppressing their ability, and maybe even the desire, to find food. However, when males were deprived of food, they produced dramatically higher levels of this receptor, allowing them to temporarily focus on finding food.

To confirm the role of these genetic differences between the sexes on behavior, the researchers designed a series of experiments in which they observed the activity of C. elegans when placed in a petri-dish and confronted with the option to either feed or go in search of a mate. The hermaphrodites were place in the center of the dish at a food source and, as expected, they stayed put.

The males were placed in their own individual food sources at the periphery of the dish. As a further obstacle between the males and their potential mates, an additional ring of food surrounded the hermaphrodites in the center of the dish. The males in the experiment consisted of two categories, one group with a normal genetic profile and another group that had been engineered by the researchers to overexpress the ODR-10 receptor, essentially making them more sensitive to the smell of food.

The researchers found that the normal worms left their food source and eventually made their way to the center of the dish where they mated with the hermaphrodites. The genetically engineered males were less successful at finding a mate, presumably because they were more interested in feeding. By examining the genetic profile of the resulting offspring, the scientists observed that the normal males outproduced the genetically engineered males by 10 to one.

“We find that modulated expression of a single chemoreceptor plays a key role in naturally occurring variation in the prioritization of feeding and exploration,” wrote the investigators. “The convergence of three independent regulatory inputs—somatic sex, age, and feeding status—on chemoreceptor expression highlights sensory function as a key source of plasticity in neural circuits.”

“These findings show that by tuning the properties of a single cell, we can change behavior,” said Dr. Portman. “This adds to a growing body of evidence that sex-specific regulation of gene expression may play an important role in neural plasticity and, consequently, influence differences in behaviors and in disease susceptibility between the sexes.”